A polarimeter is an instrument used to measure the angle of rotation caused when polarized light passes through an optically active substance. It consists of a polarimeter tube and operation panel. When light passes through a left-handed or right-handed sample, the translucent semicircular fields in the polarimeter gradually change. There are different types of polarimeters. The specific rotation, a unique property of substances, can be calculated using the measured angle of rotation, concentration, temperature, and length of the sample cell. Polarimeters are used in industries like chemistry, food, beverages, and pharmaceuticals for applications such as quality control and purity measurements.
2. AS AN INSTRUMENT
is a scientific instrument used to measure the
angle of rotation caused by passing polarized
light through an optically active substance.
8. RESULTS IN POLARIMETER
When the left semicircular field is the brighter (left
handed rotating sample) continuously press the
left handed ROTATE switch and the translucent
semi-circular fields gradually change as below:
9. RESULTS IN POLARIMETER
When the right semicircular field is the brighter
(right handed rotating sample) continuously press
the right handed ROTATE switch and the
translucent semi-circular fields gradually change
as below:
13. PROBLEM SOLVINGG
1. DATA
conc of sample - 300 grams
Length of the observation tube - 1 dm
Temperature - 25C
Wavelength of measuring light used- 589nm
Reading of the rotation of the sample- 113
Specific rotation of sample - ?
14. PROBLEM SOLVINGG
2. DATA
conc of sample - ?
Length of the observation tube - 10 cm
Temperature - 25C
Wavelength of measuring light used- 589nm
Reading of the rotation of the sample- 125
Specific rotation of sample - 145
15. PROBLEM SOLVINGG
3. DATA
Conc of sample - 10 g
Length of the observation tube - 10 cm
Temperature -
25C
Wavelength of measuring light used- 589nm
Reading of the rotation of the sample- ?
Specific rotation of sample - 110
16. SOURCE OF ERRORS
The angle of rotation of an optically active substance can
be affected by:
Concentration of the sample
Wavelength of light passing through the sample (generally,
angle of rotation and wavelength tend to be inversely
proportional)
Temperature of the sample (generally the two are directly
proportional)
Length of the sample cell (input by the user into most
automatic polarimeters to ensure better accuracy)
Filling conditions (bubbles, temperature and concentration
gradients)
Most modern polarimeters have methods for compensating
or/and controlling these errors
17. APPLICATION
Chemical industry
Many chemicals exhibit a specific rotation as a unique
property (an intensive property like refractive index or
Specific gravity) which can be used to distinguish it.
Polarimeters can identify unknown samples based on
this if other variables such as concentration and
length of sample cell length are controlled or at least
known. This is used in the chemical industry.
By the same token, if the specific rotation of a sample
is already known, then the concentration and/or purity
of a solution containing it can be calculated.
Most automatic polarimeters make this calculation
automatically, given input on variables from the user
18. APPLICATION
Food, beverage and pharmaceutical industries
Concentration and purity measurements are especially important to determine
product or ingredient quality in the food & beverage and pharmaceutical
industries. Samples that display specific rotations that can be calculated for purity
with a polarimeter include:
Steroids
Diuretics
Antibiotics
Narcotics
Vitamins
Analgesics
Amino acids
Essential oils
Polymers
Starches are the most abundant substances in nature and used in various
sectors of the food and pharmaceutical industry as well as the building sector.
Polarimetric quality control of starch therefore is important in various industries.
Sugars
19. SEATWORK
1.A sample of pure 2-butanol was placed in a 10cm
polarimeter tube. Using the D-line of a sodium
lamp, the observed rotation at 20C was a= +104°.
The conc of the compound is 0.805 g/ mL.
What is the specific rotation of 2-butanol?
20. ANSWER TO SEATWORK
1. T/D = A/ Lx C
T/D = ?
A = 104°
l = 10 cm or 1 dm
c = 0.805 g/ mL
= 104 / 1 dm X 0.805 g/ mL
= + 129°
21. SEATWORK:
2. Calculate the observed rotation of a solution of
5.245 g of 1-ammonium-1-phenylethane diluted
to a volume of 100 mL w/ a methanol at 20C
using the D-line of a sodium lamp and a 1 dm
tube.
Specific rotation of this material=(-30°)
Sample concentration is 5.245 g in 100mL
22. ANSWER TO SEATWORK
2. T/D= A/ Lx C
A = ?
T/D = -3O
L = 1.00 dm
C = 5.245 g in 100 mL
-30 = X100
1 X 5.245 g/ 100 mL
-30 (5.245) = x100
100
x =-157.35 °
23. SEATWORK:
3. Calculate the specific rotation of 2,3-tartaric acid
based on the ff observation:
A 0.856 g sample of pure acid was diluted to 10 mL
w/ water and observed in a 1.00 dm polarimeter
tube. The observed rotation using the 589 nm line
of a sodium lamp at 20C was a=+1.06°.
24. ANSWER TO SEATWORK
3. T/D= A/ Lx C
T/D = ?
A = +1.06
L = 1 dm
C = 0.856 g in 10 mL
T/D = 1.06/ 1 X 0.856 g/ 10mL
= 1.06/ 0.0856
= 12.38°
Notes de l'éditeur
Some chemical substances are optically active, and polarized (unidirectional) light will rotate either to the left (counter-clockwise) or right (clockwise) when passed through these substances. The amount by which the light is rotated is known as the angle of rotation.
The specific rotation is a physical property and defined as the optical rotation α at a path length l of 1 dm, a concentration c of 1g/100 mL, a temperature T (usually 20 °C) and a light wavelength λ (usually sodium D line at 589.3 nm):
Polarization by reflection was discovered in 1808 by Étienne-Louis Malus (1775–1812).
Polarimeters measure this by passing monochromatic light through the first of two polarising plates, creating a polarized beam. This first plate is known as the polarizer.[6] This beam is then rotated as it passes through the sample. After passing through the sample, a second polarizer, known as the analyzer, rotates either via manual rotation or automatic detection of the angle. When the analyzer is rotated to the proper angle, the maximum amount of light will pass through and shine onto a detector
Parts of the polarimeter
FRONT SIDE rear side operation panel
Eye piece power switch zero set switch
Display channel power input connector zero set ready lamp
Sample chamber cover fuse holder rotate switch
Thermosensor rating label rotate (left handed rotation) switch
Sample stage shift temp switch
Indication selector switch
The ratio, the purity, and the concentration of two enantiomers can be measured via polarimetry. Enantiomers are characterized by their property to rotate the plane of linear polarized light. Therefore, those compounds are called optically active and their property is referred to as optical rotation. Light sources such as a light bulb, a light-emitting diode (LED), or the sun emit electromagnetic light waves. Their electric field oscillates in all possible planes relative to their direction of propagation. In contrast to that, the waves of linear-polarized light oscillate in parallel planes.[3]
If light encounters a polarizer, only the part of the light that oscillates in the defined plane of the polarizer may pass through. That plane is called the plane of polarization. The plane of polarization is turned by optically active compounds. According to the direction in which the light is rotated, the enantiomer is referred to as dextrorotatory or levorotatory.
The optical activity of enantiomers is additive. If different enantiomers exist together in one solution, their optical activity adds up. That is why racemates are optically inactive, as they nullify their clockwise and counter clockwise optical activities. The optical rotation is proportional to the concentration of the optically active substances in solution. Polarimeters may therefore be applied for concentration measurements of enantiomer-pure samples. With a known concentration of a sample, polarimeters may also be applied to determine the specific rotation (α physical property) when characterizing a new substance.
Laurent's half-shade polarimeter[edit]
When plane polarised light passes through some crystals,the velocity of left polarised light is different from that of the right polarised light thus the crystals are said to have two refractive indices i.e. double refracting
Biquartz polarimeter[edit]
In biquartz polarimeters, a biquartz plate is used. Biquartz plate consists of two semi circular plates of quartz each of thickness 3.75mm. One half consists of right-handed optically active quartz,while the other is left-handed optically active quartz.
Lippich polarimeter[edit]
Quartz-Wedge polarimeter[edit]
Manual[edit]
The earliest polarimeters, which date back to the 1830s, required the user to physically rotate one polarizing element (the analyzer) whilst viewing through another static element (the detector). The detector was positioned at the opposite end of a tube containing the optically active sample, and the user used his/her eye to judge the "alignment" when least light was observed. The angle of rotation was then read from a simple protractor fixed to the moving polariser to within a degree or so.
Although most manual polarimeters produced today still adopt this basic principle, the many developments applied to the original opto-mechanical design over the years have significantly improved measurement performance. The introduction of a half-wave plate increased "distinction sensitivity", whilst a precision glass scale with vernier drum facilitated the final reading to within ca. ±0.05º. Most modern manual polarimeters also incorporate a long-life yellow LED in place of the more costly sodium arc lamp as a light source.
Semi-automatic[edit]
Today, semi-automatic polarimeters are available. The operator views the image via a digital display adjusts the analyzer angle with electronic controls.
Fully automatic[edit]
Fully automatic polarimeters are now available and simply require the user to press a button and wait for a digital readout. Fast automatic digital polarimeters yield an accurate result within a second, regardless of the rotation angle of the sample. In addition, they provide continuous measurement, facilitating High-performance liquid chromatography and other kinetic investigations.
Another feature of modern polarimeters is the Faraday modulator. The Faraday modulator creates an alternating current magnetic field. It oscillates the plane of polarization to enhance the detection accuracy by allowing the point of maximal darkness to be passed through again and again and thus be determined with even more accuracy.
As the temperature of the sample has a significant influence on the optical rotation of the sample, modern polarimeters have already included Peltier Elements to actively control the temperature. Special techniques like a temperature controlled sample tube reduce measuring errors and ease operation. Results can directly be transferred to computers or networks for automatic processing.[7] Traditionally, accurate filling of the sample cell had to be checked outside the instrument, as an appropriate control from within the device was not possible. Nowadays a camera system allows accurate monitoring of the sample and filling conditions in the sample cell from inside the instrument. A telecentric camera gives a sharp image over the complete length of any sample cell placed within modern instruments. The online monitoring of the filling process ensures that no bubbles or particles obstruct the measurement. A picture can be saved together with the recorded data. Any temperature gradients, inhomogeneous sample distributions or air bubbles can immediately be recognized before measurement, so that potential errors caused by bubbles or particles are no longer an issue.
Laurent's half-shade polarimeter[edit]
When plane polarised light passes through some crystals,the velocity of left polarised light is different from that of the right polarised light thus the crystals are said to have two refractive indices i.e. double refracting
Biquartz polarimeter[edit]
In biquartz polarimeters, a biquartz plate is used. Biquartz plate consists of two semi circular plates of quartz each of thickness 3.75mm. One half consists of right-handed optically active quartz,while the other is left-handed optically active quartz.
Lippich polarimeter[edit]
Quartz-Wedge polarimeter[edit]
Manual[edit]
The earliest polarimeters, which date back to the 1830s, required the user to physically rotate one polarizing element (the analyzer) whilst viewing through another static element (the detector). The detector was positioned at the opposite end of a tube containing the optically active sample, and the user used his/her eye to judge the "alignment" when least light was observed. The angle of rotation was then read from a simple protractor fixed to the moving polariser to within a degree or so.
Although most manual polarimeters produced today still adopt this basic principle, the many developments applied to the original opto-mechanical design over the years have significantly improved measurement performance. The introduction of a half-wave plate increased "distinction sensitivity", whilst a precision glass scale with vernier drum facilitated the final reading to within ca. ±0.05º. Most modern manual polarimeters also incorporate a long-life yellow LED in place of the more costly sodium arc lamp as a light source.
Semi-automatic[edit]
Today, semi-automatic polarimeters are available. The operator views the image via a digital display adjusts the analyzer angle with electronic controls.
Fully automatic[edit]
Fully automatic polarimeters are now available and simply require the user to press a button and wait for a digital readout. Fast automatic digital polarimeters yield an accurate result within a second, regardless of the rotation angle of the sample. In addition, they provide continuous measurement, facilitating High-performance liquid chromatography and other kinetic investigations.
Another feature of modern polarimeters is the Faraday modulator. The Faraday modulator creates an alternating current magnetic field. It oscillates the plane of polarization to enhance the detection accuracy by allowing the point of maximal darkness to be passed through again and again and thus be determined with even more accuracy.
As the temperature of the sample has a significant influence on the optical rotation of the sample, modern polarimeters have already included Peltier Elements to actively control the temperature. Special techniques like a temperature controlled sample tube reduce measuring errors and ease operation. Results can directly be transferred to computers or networks for automatic processing.[7] Traditionally, accurate filling of the sample cell had to be checked outside the instrument, as an appropriate control from within the device was not possible. Nowadays a camera system allows accurate monitoring of the sample and filling conditions in the sample cell from inside the instrument. A telecentric camera gives a sharp image over the complete length of any sample cell placed within modern instruments. The online monitoring of the filling process ensures that no bubbles or particles obstruct the measurement. A picture can be saved together with the recorded data. Any temperature gradients, inhomogeneous sample distributions or air bubbles can immediately be recognized before measurement, so that potential errors caused by bubbles or particles are no longer an issue.
t/d = specific rotation of the substance determined at 25Cusing the D line of Na
A = relative optical rotation
L = length of the observation tube (dm)
100cm = 1 m
1m = 10 dm
C = conc of the optically active matters in sample
T = temperature
D = symbol to show the wavelength of measuring light D ray= 589 nm.
CALIBRATION
Polarimeters can be calibrated – or at least verified – by measuring a quartz plate, which is constructed to always read at a certain angle of optical rotation (usually +34°, but +17° and +8.5° are also popular depending on the sample). Quartz plates are preferred by many users because solid samples are much less affected by variations in temperature, and do not need to be mixed on-demand like sucrose solutions
polarimeter can be used to identify which isomer is present in a sample – if it rotates polarized light to the left, it is a levo-isomer, and to the right, a dextro-isomer. It can also be used to measure the ratio of enantiomers in solutions.
The optical rotation is proportional to the concentration of the optically active substances in solution. Polarimetry may therefore be applied for concentration measurements of enantiomer-pure samples. With a known concentration of a sample, polarimetry may also be applied to determine the specific rotation (a physical property) when characterizing a new substance
